In PWR, neutron absorbing rods are usually grouped in control clusters. Among the control clusters of a nuclear reactor, some are dedicated to controlling the core reactivity in normal operation of the nuclear reactor. They may be frequently displaced in stepping mode to insert the neutron absorbing rods into or extracted them from the guide thimbles of the nuclear fuel assemblies of the nuclear reactor. Stepping motions may cause distributed wear on the neutron absorbing rods due to contact with guides against which they rub.
The other control clusters stay stationary in the guides in the upper part of the nuclear reactor during normal operation. When a rapid return to the sub-critical state is required, these clusters are simultaneously and fully inserted by gravity in the reactor core, i.e. the neutron absorbing rods are inserted in the corresponding guide thimbles of the corresponding nuclear fuel assemblies. Flow induced vibrations may cause localized wear on the neutron absorbing rods of the stationary control clusters:                at the contact levels in the guides, and        at their lower end due to contact of the lower portion of the cladding and the lower end plug with the upper portion of the guide thimbles of the nuclear fuel assembly (“tip-wear”).        
The possible consequences of wear, whether from stepping motions or flow-induced vibrations, are:                cladding wear-through with potential contamination of the water of the primary coolant system of the nuclear reactor by the neutron absorbing material enclosed in the cladding, and        neutron absorbing rod mechanical failure due to reduction of the mechanical strength of the cladding.        
The frequency and amplitude of the movements of some of the control clusters, especially when the reactor is operated in the load follow mode (suivi de charge in French), and the frequency and amplitude of vibration of some of the neutron absorbing rods, especially for control clusters remaining in a stationary position, are such that it is necessary to check frequently and often prematurely replace a number of control clusters given the wear resulting from friction.
To prevent this wear, it was proposed to harden the outer surface of the claddings through nitriding and/or carburizing treatments. U.S. Pat. No. 4,873,117, EP-446,083, EP-537,062 and EP-801,142 describe the steps of such treatments especially under plasma conditions.
Such treatments can efficiently protect the claddings and the lower end plugs of neutron absorbing rods against wear and corrosion.
As disclosed e.g. in U.S. Pat. No. 4,873,117, the claddings to be treated are first cleaned, then fitted with their lower plugs. A thermal buffer body, which can be constituted by a slug of stainless steel, is placed in each cladding and then a temporary upper plug is fixed on each cladding. The temporary upper plug is used for holding the cladding during treatment and temporarily closing off the open end of the cladding and then avoiding pollution of the treatment atmosphere by air contained in the cladding. The claddings are placed in a treatment enclosure and their upper portions close to the upper plugs are advantageously masked.
After treatment, the claddings are taken out of the enclosure. The temporary upper plugs are removed, and the claddings are loaded with neutron absorbing material and a final upper plug is welded on the free end of each cladding.
Masking of the upper portion of the cladding avoids treatment of the cladding material in this zone. Indeed, such a treatment would have an impact on the material features and on the properties of this upper portion which is to be welded to the final upper plug. As an example, carbonitriding the upper portion might lead to carbide and/or nitride precipitates during welding to the upper plug, thus leading to a lower resistance of the cladding to strain and corrosion.
More generally portions of parts which have to be treated, e.g. oxidized, nitrided, carbonitrided . . . shall be masked so as to avoid modifying material features and properties and hindering the subsequent manufacturing operations applied to these portions: mechanical forming, stamping, welding, mechanical machining, threading . . . .
Various masking methods are known and used for shielding portions of parts which need to be treated.
In particular, as described above, solid masks have been used for treatment involving plasmas. Such a solid mask receives a portion to be shielded from treatment with a fitting clearance. This clearance corresponds to the space needed to ensure the correct mounting of the mask on the portion and needs to have a thickness lower than the Debye length. The Debye length is the scale over which mobile charge carriers (e.g. electrons) screen out electric fields for specific plasma conditions. In other words, the Debye length is the distance under which significant charge separation can not occur.
The thickness of the clearance being lower than the Debye length, the plasma conditions are not present in the clearance, so that a part portion surrounded by the solid mask with said clearance will be shielded from the treatment applied to the rest of the part located outside the solid mask.
However, such solid masks are inefficient if the nominal fitting clearance, which can not be lower than the cumulated manufacturing tolerances of the masks and of the portions to be shielded, is greater than the Debye length. In addition, even if the nominal fitting clearance is lower than the Debye length, the solid masks have proved to be inefficient at least for plasma carbonitriding treatments, the masked portion experiencing depassivation and/or hardening despite the presence of the solid mask.
Solid masks with clearance have also proved to be inefficient for oxygen diffusion treatments, in the plasma or gas phases, of hafnium claddings or hafnium solid bars used as neutron absorbing rods in control clusters, or of zirconium alloy claddings used for nuclear fuel rods as taught for instance in WO-2009/081013.
WO-02/066,698 discloses a solid mask used to shield a portion of a part from a carburizing treatment. The solid mask has a thermal expansion coefficient lower than the thermal expansion coefficient of the part to be treated. Thus, the clearance between the portion to be shielded and the solid mask will decrease during the carburizing treatment. However, this requires high treatment temperatures, e.g. around 500° C. to 900° C. depending on the materials of the part and of the mask, and there is a high risk of damaging the outer surface of the part, or even to modify its geometry if the part is a thin-walled tube, through contact with said mask.
This solid mask can therefore not be used e.g. for plasma carbonitriding of neutron absorbing rod claddings. Indeed, the materials used for the neutron absorbing rod claddings (mainly stainless steels or nickel-based alloys) have thermal expansion at the treatment temperatures which are lower than or at best of the same order of magnitude than the manufacturing tolerances of the claddings. There is thus no guarantee that the masked portions of all the claddings of the treated batch are protected in a homogeneous manner. Moreover, the outer surfaces of the claddings shall not be damaged by fixing or removal of the masks or during the treatment, which prevents the use of smaller clearance.